gst-validate-scenario: Add GST_VALIDATE_SCENARIO_EOS_HANDLING_LOCK
There was a race in appsrc-push when the pushed buffer caused an EOS. The EOS event could be handled by the main thread, finishing the test while the action, executing in the streaming thread, has not finished yet.
A mutex is now introduced to add mutual exclusion for the two threads so that an EOS does not cause the termination of the test while the action is still going.